Optically controlled resonant tunnel diode oscillator

ABSTRACT

An optically controlled resonant tunnel diode oscillator assembly having aesonant tunnel diode (RTD) which, when voltage biased, oscillates at a free running frequency; an optical signal delivery system, such as a light intensity modulator connected to optical fibers; and other oscillator circuitry which one skilled in the art could readily adapt to the concepts of the present invention. In operation, the free running oscillation of the RTD can be frequency modulated or can be intensity locked to the intensity modulated optical signal delivered via the optical signal delivery system.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used and licensed byor for the Government of the United States for governmental purposeswithout the payment to us of any royalties thereon.

CROSS REFERENCE

This application is related to U.S. Pat. No. 5,144,261 issued Sep. 1,1992 application No. 07/821,729 entitled, "OPTICALLY INJECTION LOCKEDRESONANT TUNNEL DIODE OSCILLATOR," filed by the inventors herein on Jan.15, 1992.

FIELD OF THE INVENTION

The invention described herein relates to optically controlledoscillators, and more particularly to optically controlled frequencymodulated or injection locked oscillators which utilize resonant tunneldiodes.

BACKGROUND OF THE INVENTION

A growing interest has developed for the direct optical control ofsemiconductor devices due to the potential applications in phased arrayradar and communications. The following list of documents comprises asample of articles (1-6) which concern the optical control of suchdevices, and a sample of articles (7-14) which concern the design andfabrication of resonant tunnel diodes.

1. A. S. Daryoush, "Optical Synchronization of Millimeter-WaveOscillators for Distributed Architectures," IEEE Trans. Microwave TheoryTech., Vol. 38, pp. 467-476, May 1990; and a related article by, K.Kurokawa, "Injection Locking of Microwave Solid-State Oscillators,"Proc, IEEE 61, 1386 (1973).

2. A. J. Seeds and A. A. A. DeSalle, "Optical Control of MicrowaveSemiconductor Devices," IEEE Trans. Microwave Theory Tech., Vol. 38, pp.577-584, May 1990.

3. T. C. L. G. Sollner, E. R. Brown and H. Q. Le, "Microwave andMillimeter-Wave Resonant-Tunneling Devices," Lincoln Lab. Jour., Vol. 1,pp. 89-105, 1988.

4. S. C. Kan, S. Sanders, G. Griffel, G. H. Lang, S. Wu, and A. Yariv,"Optical Switching of a New Middle Trace in an Optically ControlledParallel Resonant Tunneling Device-Observation and Modeling," Appl.Phys. Lett., Vol. 58, pp. 1548-1550, 1991.

5. P. England, J. Yee, L. T. Florez, J. P. Harbison and J. E. Golub,"Optical Switching in Resonant Tunneling Structures," Conference onQuantum Electronics Laser Science, May 12-17, 1991, Baltimore, Md., 1991Technical Digest Series, Vol. 11, p. 34, 1991.

6. D. J. Struzbecher, J. F. Harvey, T. P. Higgins, A. C. Paolella, andR. A. Lux, "Direct Optical Frequency Modulation of a Resonant TunnelDiode Oscillator," submitted to IEEE Electron Device Lett., 29 Jul.1991.

7. L. L. Chang, L. Esaki and R. Tsu, "Resonant Tunneling inSemiconductor Double Barriers," Appl. Phys. Lett., vol. 24, pp. 593-595,1974.

8. I. Mehdi, R. K. Mains, and G. I. Haddad, "Effect of Spacer LayerThickness on the Static Characteristics of Resonant Tunneling Diodes,"Appl. Phys. Lett. 57, 899 (1990); and a related article by J. E. Oh, I.Mehdi, J. Pamulapati, P. K. Bhattacharya, and G. I. Haddad, "The Effectof Molecular Beam Epitaxial Growth Conditions on the ElectricalCharacteristics of In₀.52 Al₀.48 As/In₀.53 Ga₀.47 As Resonant TunnelingDiodes," J. Appl. Phys. 65, 842 (1989); I. Mehdi, R. K. Mains, G. I.Haddad, and U. K. Reddy, "Properties and Device Applications of DeepQuantum Well Resonant Tunneling Structures," Surf. Sci. 228, 426 (1990);and also by the same authors, R. K. Mains, I. Mehdi, and G. I. Haddad,"Effect of Spatially Variable Effective Mass on Static and DynamicProperties of Resonant Tunneling Devices," Appl. Phys. Lett. 55, 2631(1989).

9. C. J. Arsenault and M. Meunier, "Proposed New Resonant TunnelingStructures with Impurity Planes of Deep Levels in Barriers," J. Appl.Phys. 66, 4305 (1989).

10. R. L. Wang, Y. K. Su, Y. H. Wang, and K. F. Yarn, "NegativeDifferential Resistance of a Delta-Doping-Induced Double BarrierQuantum-Well Diode at Room Temperature," IEEE Electron Dev. Lett. 11,428 (1990).

11. K. Reddy, A. J. Tsao, S. Javalagi, G. K. Kumar, D. R. Miller, and D.P. Neikirk, "Quantum well injection transit time (QWITT) diodeoscillators," in Conference Digest Fifteenth International Conference onInfrared and Millimeter Waves, 10-14 Dec 1990, Orlando, Fla., ed. R. J.Temkin, (SPIE vol. 1514), p. 88.

12. J. M. Gering, T. J. Rudnick, and P. D. Coleman, "Microwave DetectionUsing the Resonant Tunneling Diode," IEEE Trans. Microwave Thry. andTech. 36, 1145 (1988).

13. T. C. L. G. Sollner, P. E. Tannenwald, D. D. Peck, and W. D.Goodhue, "Quantum Well Oscillators," Appl. Phys. Lett. 45, 1319 (1984);and a related article by, E. R. Brown, T. C. L. G. Sollner, W. D.Goodhue, and C. D. Parker, "Millimeter-Band Oscillations Based onResonant Tunneling in a Double-Barrier Diode at Room Temperature," Appl.Phys. Lett. 50, 83 (1987).

14. G. Keiser, Optical Fiber Communications, (McGraw-Hill, N.Y., 1983)and see C. K. Kao, Optical Fiber Systems: Technology, Design, andApplications, (McGraw, 1982).

As is evident from the above cited references, the technology ofbuilding resonant tunnel diodes (RTDs) is well established. As well, thetechnology of microwave light generation and delivery using lasers,light emitting diodes, plasma tubes and the like in combination withintensity modulators and fiber optics is also well known. Therefore, asis suggested by the above identified references, one skilled in the artwould readily be able to design any number of optical or integratedoptic systems to deliver light intensity modulated at microwave ormillimeter wave frequencies in the nominally 2 mW power range withphoton energies above the bandgap of a semiconductive device material tothe semiconductive device and incorporate such a semiconductive devicein oscillator circuit applications.

An example of important technical fields where such semiconductordevices would be able to be directly incorporated is in phased arrayradar and communication systems and in remote antenna systems. Assuggested by references 1 and 2, intensity modulated light wouldmodulate active oscillator modules using these semiconductor devices anddistributed over an antenna array remote from the rest of the radar orcommunication system. One means of achieving modulation, as suggested byreference 2, is by modulating oscillators with a direct optical signaldelivered over optical fibers. Reference 2 describes the various meanswhich are available to optically control semiconductor devices,including the direct optical modulation of oscillators utilizingIMPATTS, FETs, and HEMTs. However, these semiconductor devices lack someof the enhanced performance characteristics of RTDs and, therefore,modulation or locking of oscillator modules would be further optimizedby optically controlling RTDs incorporated in such oscillators.

The present invention addresses this present need for direct opticalcontrol of an RTD to modulate or lock an RTD oscillator.

SUMMARY OF THE INVENTION

One object of the invention is to modulate or lock the natural frequencyof a semiconductor oscillator with minimum noise using an externallymodulated optical signal.

Another object of the invention is to provide this control using anexternal signal which can be easily transmitted over a media which isnot influenced by other electrical signals and which is light weight andinexpensive.

In accordance with these objectives, the present invention provides anoscillator based on a resonant tunneling structure whose naturaloscillating frequency is frequency modulated or locked by an externalintensity modulated optical signal.

In the most generic embodiment of the present invention, the opticallycontrolled, frequency modulated or injection locked oscillator comprisesa resonant tunnel diode (RTD) which, when voltage biased, oscillates ata free running frequency; an optical signal delivery system, such as alight intensity modulator connected to optical fibers; and otheroscillator circuitry which one skilled in the art could readily adapt tothe concepts of the present invention. In operation, the free runningoscillation of the RTD can be frequency modulated or injection locked tothe frequency of the intensity modulated optical signal delivered viathe optical signal delivery system. This injection locking occurs as themodulation frequency approaches the free running oscillation frequency.In experiments conducted by the inventors herein, injection locking ofthe oscillator occurred over a bandwidth of 150 Kilohertz with a notedreduction in FM noise; and frequency modulation of the oscillatoroccurred at modulation frequencies as high as 100 MH_(z).

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and details of the present inventionwill become apparent in light of the ensuing detailed disclosure, andparticularly in light of the drawings wherein:

FIG. 1 is a schematic circuit diagram of an optically modulated resonanttunnel diode oscillator assembly according to the present invention;

FIG. 2 is an equivalent circuit diagram of a portion of FIG. 1;

FIG. 3 is an edge elevation view of an embodiment corresponding to thecircuit diagram of FIG. 1;

FIG. 4 is a partial planar view as taken along the line 4--4 of FIG. 3;

FIG. 5 is a schematic block diagram of a test circuit used as an initialdemonstration of the direct optical frequency modulation and injectionlocking of an RTD oscillator as embodied by the present invention;

FIG. 6 is a curve of output power versus frequency showing the powerspectrum of the unmodulated and unlocked RTD oscillator;

FIG. 7 is a curve of output power versus frequency of the oscillatorsubassembly portion of FIG. 5 showing the power spectrum of the unlockedoscillator together with the optically injected signal which is shown asthe left most peak;

FIG. 8 is a curve of the output power versus frequency for theoscillator subassembly portion of FIG. 5 for showing the power spectrumof the injection locked oscillator;

FIG. 9 is a superimposition of the curves from FIGS. 6 and 8;

FIG. 10 is a plot of the power spectrum versus frequency of the RTDoscillator with the laser light on and off;

FIG. 11 is a plot of the output power spectrum versus frequency of theRTD oscillator when being frequency modulated using laser light which isintensity modulated at 1 KH_(z) ; and

FIG. 12 is a plot of the output power spectrum versus frequency of theRTD oscillator when being frequency modulated using laser light which isintensity modulated at 75 MH_(z).

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIGS. 1, 2, 3 and 4, a circuit or assembly 10 is provided.Assembly 10 includes an input light emitter subassembly 11 and anoscillator subassembly 12. Assembly 10 may also have an optional add-onoutput power sensor or test subassembly (not shown) which would be partof the load impedance Z_(L) 25.

As shown in FIGS. 1 and 2, oscillator subassembly 12 receives modulatedlight from emitter subassembly 11. Incorporated in oscillatorsubassembly 12 is the resonant tunnel diode 22. FIG. 2 shows anapproximate equivalent circuit of the RTD 22 with a shunt capacitanceC_(d) 43 caused by the charge separation in the RTD layers; a negativeshunt resistance -R_(d) 42 caused by the negative differentialresistance portion of the RTD I-V characteristic; and a seriesresistance r_(d) 44 caused by the resistance of the contacts, of thesemiconductor layers on each side of the tunneling structure, and of thesubstrate. Oscillator subassembly 12 also has a transmission linepropagation circuit 24, which terminates in a load impedance 25; a biasvoltage 27; and an AC isolation circuit 28 for the DC bias voltage. Inoperation, the transmission line 24 matches the load impedance Z_(L) 25to the impedance of the RTD 22. The load impedance Z_(L) 25 produces aVs output 26 and a variable impedance to tune the oscillator and thus,load impedance Z_(L) would obviously have to include any other necessaryload circuits, such as subsequent amplifier stages or an antennacircuit.

As shown in FIGS. 3 and 4, emitter subassembly 11 has a modulated laser29 and an optical fiber 30 which projects light 21. RTD diode 22 has aninterface 31 with an opening for an optical signal from the opticalfiber 30. As shown in structural detail in FIG. 3, oscillatorsubassembly 12 may comprise an upper metal strip of transmission line32, a metal ground strip or plane of transmission line 33, atransmission line dielectric or dielectric layer 34 sandwiched betweenmetal strips 33 and 34 and a ground plane 38 attached to the bottomportion of metal ground strip 33. RTD diode 22 is connected totransmission line 32 and ground plane 33 via ohmic contacts 35 and 36.The circuit is completed by grounding ground plane 33 via transmissionline ground connection 37.

FIG. 5 is explained hereafter.

In operation, RTD 22 is biased in the negative differential resistance(NDR) region. When so biased, the RTD oscillates at a free-runningfrequency which is determined by the internal and external capacitancesand the external inductance of the device. The free-running oscillationof RTD 22 is then frequency modulated or injection locked by theintensity modulated optic signal 21 which is delivered via subassembly11. Intensity modulation of the optical signal results in frequencymodulation of the RTD oscillator.

When tested, it was found that, as the modulation frequency in theexperiment approached the free running oscillator frequency, injectionlocking over a bandwidth of 150 kilohertz occurred with a markedreduction in FM noise.

FIG. 6 shows the free running signal (at 2.7574 GH_(z)) of the RTDoscillator with no modulating signal and with no injected signal.

FIG. 7 shows the RTD oscillator signal with an optically injectedsignal, as previously described.

FIG. 8 shows the oscillator signal after it has locked to the injectedsignal.

FIG. 9 shows a superimposition of the signals from FIG. 6 and FIG. 8which compares the free running oscillator signal to the injectionlocked oscillator signal. As shown in FIG. 9, the injection lockedsignal has lower FM noise and a narrower frequency spread centered onthe injected signal frequency. This behavior is typical of injectionlocked oscillators.

FIG. 10 shows the shift in frequency of the RTD oscillator as theinjected laser light is turned on and off.

When the intensity modulated laser light is injected into the RTD, theresulting frequency shifts are responsible for the frequency modulationof the RTD oscillator.

FIG. 11 shows the frequency modulated oscillator signal when the RTD isbeing modulated by light which is intensity modulated at 1 KH_(z). Notethat for this figure the free running oscillator frequency was 1.2762GH_(z). For this relatively small modulation frequency, the modulationindex is very large.

FIG. 12 shows the frequency modulated oscillator signal when the RTD isintensity modulated at 75 MH_(z). For this modulation frequency, themodulation index is small.

The oscillator circuit, the RTD structure, and the optical coupling wereall non-optimized. Therefore, the curves in FIGS. 6-12 demonstrate thephysical effect and they do not depict the optimum conditions orlimitations of this technique.

The physical mechanisms responsible for this behavior involve thegeneration by the absorbed light of electron-hole pairs. Light absorbedin the GaAs material (primarily in the substrate) creates electron-holepairs, changing the carrier density, and hence the series resistance(r_(d) in FIG. 2) of the RTD at the same frequency as the variations inthe light intensity. The varying r_(d) causes a varying current in theRTD, changing the operating point on the IV characteristic at the samefrequency as the modulated light signal. There will also be acontribution to the injected signal from holes generated by the absorbedlight in the substrate, in the barriers, in the GaAs layers on each sideof the barriers, and in the quantum well. These holes become trapped atthe RTD interfaces, causing a change in the voltage drop across eachlayer of material in the RTD and therefore a change in the operatingpoint. The total overall injected signal causes modulation of thefrequency of the RTD oscillator.

As shown in FIG. 5, which is the schematic diagram of the initial testcircuit, laser 29 has a laser diode 51, a laser bias unit 52, and an RFgenerator 53. Optical fiber 30 connects at a first end thereof to laserdiode 51 and directs light beam 21 at the other end thereof onto RTDdiode 22. RTD diode 22 has a top portion or diode mesa 54 which receiveslight 21, and has a bond wire 55 which connects to strip 32, and has acoplanar fixture 56. The test circuit also includes a slide screw tuner57, a bias tee 58, an RTD bias unit 59, and a spectrum analyzer 60.Tuner 57 has an inboard coaxial line 61, which connects to RTD diode 22,and has an outboard coaxial line 62 which connects to bias tee 58.Analyzer 60 has an input conductor 63 which connects to bias tee 58. RTDbias 59 has an output conductor 64 which connects to bias tee 58.

The advantages of assembly 10 are indicated hereafter.

Assembly 10 has the potential for external optical modulation of thefrequency of a semiconductor oscillator 12 operating at higher frequencyand with less noise than oscillators constructed from othersemiconductor devices.

Since resonant tunnel diodes (RTDs) have demonstrated operation atfrequencies as high as 700 Gigahertz with very low noise, it is expectedthat frequency modulated oscillators, in addition to injection lockedoscillators can be built which operate at higher frequencies and withlower noise than oscillators using other semiconductor devices.

Assembly 10 can be used in various applications, including phase arrayradar, phased array communication systems, remote control of microwaveand millimeter wave antennas, EMI/EMP hardening by optical devices forlow power signal processing, and optical isolation of microwavesubsystems to reduce vulnerability to reflected microwave power and topermit separate shielding from EMI and EMP.

While the invention has been described in its preferred embodiment, itis to be understood that the words which have been used are words ofdescription rather than words of limitation and that changes may be madewithin the purview of the appended claims without departing from thetrue scope and spirit of the invention in its broader aspects.

For example, other ways can be used to couple the light into the RTD 22.Other circuits can be used to propagate the output signal, such astransmission line, waveguide, or resonators of various geometries. Otherways can be used to connect the propagation circuit to the RTD 22. Otherways can be used to terminate the propagation circuit in a load. Andother ways can be used to isolate the DC bias circuit from the ACcircuit. As well, various materials can be used for substrate 38.

What is claimed is:
 1. A frequency modulated oscillator comprising:anintensity modulated optical signal delivery system; oscillatorcircuitry; and a resonant tunnel diode incorporated in the oscillatorcircuitry, the resonant tunnel diode being exposed to an intensitymodulated optical signal delivered by the optical signal deliverysystem; whereby the resonant tunnel diode is biased such that itoscillates at a free running frequency and the free running oscillationof the resonant tunnel diode is capable of being frequency modulated bythe intensity modulated optical signal.
 2. The frequency modulatedoscillator of claim 1 wherein the intensity modulated signal has aphoton energy level near or above the band gap energy level of thematerial comprising the resonant tunnel diode.
 3. A frequency modulatedoscillator comprising:a light source; a light intensity modulatoroptically connected to the light source; at least one optic fiberoptically connected to the light intensity modulator; oscillatorcircuitry; and a resonant tunnel diode incorporated in the oscillatorcircuitry, the resonant tunnel diode being optically coupled to at leastone optic fiber such that an intensity modulated light signal producedby the light source and modulated by the light intensity modulator maybe absorbed by the resonant tunnel diode; whereby the resonant tunneldiode is biased such that it oscillates at a free running frequency andthe free running oscillation of the resonant tunnel diode is capable ofbeing frequency modulated by the intensity modulated optical signal. 4.The frequency modulated oscillator of claim 3, wherein the oscillatorcircuitry has first and second transmission lines which have respectivefirst and second metal strip portions that are separated by a dielectriclayer.
 5. The frequency modulated oscillator of claim 3, wherein saidlight source is a laser diode.
 6. The frequency modulated oscillator ofclaim 4, wherein an end portion of the first metal strip portion formsan ohmic contact with a top portion of the resonant tunnel diode, andwherein an end portion of the second metal strip portion forms an ohmiccontact with a base portion of the resonant tunnel diode.
 7. Thefrequency modulated oscillator of claim 3, wherein the oscillatorcircuitry includes a slide screw tuner for tuning the free runningoscillation of the resonant tunnel diode; and includes a spectrumanalyzer for display; and includes a bias tee coupled to a bias voltageunit for bias voltage to the resonant tunnel diode.
 8. An opticallyfrequency modulated resonant tunnel diode oscillator assemblycomprising:an intensity modulated light emitter having an optical fiber;oscillator circuitry having a resonant tunnel diode which has a mesaportion for receiving a light beam from the optical fiber and which hasa base portion; said oscillator circuitry having a circuit portionconnected between the mesa portion and the base portion for driving theresonant tunnel diode at a free running frequency; and means connectedto the input light emitter for setting a selective value of modulationfrequency and depth of modulation of an optical signal emitted from saidinput light emitter to said mesa portion resulting in a selective valueof modulation frequency and modulation index of the frequency modulatedresonant tunnel diode oscillator.
 9. A method of frequency modulating afree running frequency of a resonant tunnel diode oscillator includingthe steps of:generating a light signal which is intensity modulated;passing the light signal through an optical fiber and out from an endthereof; directing the light signal from the optical fiber onto asurface portion of the resonant tunnel diode; causing a free runningfrequency to pass from a first portion to a second portion of a resonanttunnel diode; and causing the free running frequency of the RTD to befrequency modulated by the optical signal.